Acute deep venous thrombosis of the lower extremity is a common disorder. Traditional treatment with anticoagulation therapy is effective in reducing the associated risk of pulmonary embolism, but is ineffective in restoring patency of the venous system of the lower extremity. While systemic thrombolytic therapy has been shown to be more effective than anticoagulation, catheter-directed local thrombolytic therapy is the most effective treatment in restoring venous patency. Current treatment regimens are based on use of urokinase, infused continuously through catheters imbedded into the thrombus. These treatment regimens require doses on the order of 10 million units of urokinase, resulting in significant bleeding complications and prohibitive cost. Experience at NIH with pulse-spray treatment of axillary subclavian venous thrombosis with rtPA indicates that this is a highly effective and safe alternative thrombolytic regimen. The proposed protocol is designed to evaluate the efficiency, safety, and doses of rtPA associated with pulse-spray directed rtPA treatment of the more extensive venous thrombosis encountered in the lower extremity.